The last Ice Age ended roughly 10,000 years ago, submerging all of the Earth's coastal regions under 300-400 feet of seawater.
It is not too crazy to imagine that what today form the continental shelves of our oceans were once populated by human beings, considering that most contemporary humans tend to live along or near to the oceans' coasts.
Is it crazy to think that the remains of super ancient civilizations can be found buried along today's continental shelves? Graham Hancock states here that, "Archeology is not doing enough to explore submerged lands."
Video (about 44 mins):